Let me start this out by making something clear: competitive reasoning has no place in this discussion. In fact, if you even can make a competitive justification, the ability probably is probably too competitive to even be considered.
Any ability we choose here should be an inferior option to both of its existing abilities. Yes, this includes Infiltrator. I don't care if you think it is a weak ability, Infiltrator was chosen for competitive reasons, and any flavor ability may not outclass a competitive one. Now, this doesn't mean the ability must have no effect, but if any set not using a berry would want it over Infiltrator, it is probably too strong. With that in mind I believe that Gluttony, where any set that runs it would always be overshadowed by a similar one with Harvest, and Pickpocket, which has no real competitive use at all, are fine choices. However, things like Pressure, Defiant, Cursed Body, Unburden, Analytic and others are very much competitive and would certainly outclass the chosen competitive abilities on certain set types, and as such should not be in consideration.
Now, with that out of the way I personally would like to support Pickpocket. It has great precedence, being a DW only ability that is solely on Dark type families, and it also has good flavor in my mind for a tricky Pokemon like Malaconda. Gluttony, on the other hand, I am not a fan of. It gets the instant "omg sin!!!!" reaction, but when you think about the flavor background of Malaconda, it doesn't make much sense. Looking to the whole garden of eden serpent thing, it should want YOU to eat the fruit, and stuffing its face itself kinda defeats the purpose.
